Synthetic Mica (also called fluorphlogopite) is man-made or manufactured in a factory, as opposed to naturally mined micas. This is also why Synthetic Mica's are more expensive to produce.
Synthetic mica is a more pure and brighter powder, giving a superior clairty of colour. Natural mica, whilst a naturally off-white colour, needs to have various dyes and pigments added to it to achieve the different tones and colours. These colours are generally not natural.
It should be noted that any powder with a micron size higher than 150, in Mica or Synthetic Powder, should be cautious when using around the mucuous membrane areas, as may cause irritation.